In spark ignition type internal combustion engines (hereinafter also referred to as engines), including automobile engines, a technique which attempts to save fuel consumption and also prevent the excessive temperature rise of an exhaust gas purifying catalyzer due to excess hydrocarbon (HC) by cutting fuel injection from the fuel injection valve to cut supply of fuel to the combustion chamber has recently been developed.
Such a technique is also called fuel cut control. This fuel cut control can be performed only under certain conditions, because inevitably it causes a sudden reduction in the engine output (or, specifically an increase in the engine brake). In other words, in the case of low engine speeds, the fuel cut control cannot be carried out because there is the possibility that engine stall will take place. Of course, similarly, where a driver does not desire deceleration, the fuel cut control cannot be carried out.
Hence, in the case where the throttle valve is fully closed when the engine speed is greater than a predetermined speed, generally the fuel cut control is attempted to be performed.
In this case, for conditions for the opening degree of the throttle valve, in addition to a technique for directly detecting and judging a throttle opening degree, there is a technique for judging based on whether or not an engine is in an idle state.
In other words, the case where an engine goes to an idle state is one where the throttle valve is less than a predetermined micro opening degree, and generally, idle control is performed based on information from an idle switch which operates according to the throttle opening degree. If the throttle valve is less than a predetermined micro opening degree, it means this case nearly coincides with the case where the throttle valve is fully closed. Hence, the technique that causes fuel cut control to be carried out in the case where an engine goes to an idle state when the engine speed is greater than a predetermined speed has been developed.
However, where fuel cut control is carried out based on whether or not an engine is in an idle state, the fuel cut control is prohibited for a predetermined period after the engine has made a transition from a non-idle state to an idle state, for the purpose of confirming that the throttle valve has been fully closed.
Also, if fuel cut is performed, a sudden reduction in the output torque of an engine will be caused. In automobile engines, since a vehicle will be considerably shocked, another technique has been developed. Even if the accelerator pedal is off, the throttle valve will not be fully closed suddenly but will be gently closed by operation of a so-called dashpot, thereby avoiding a sudden reduction in the engine output torque and a great shock to a vehicle.
The aforementioned fuel cut control, incidentally, has been applied to a multipoint injection (MPI) engine equipped with fuel injection valves each in an intake port of each cylinder, but, since an in-cylinder injection engine (in-cylinder injection internal combustion engine) with a fuel injection valve faced so as to inject fuel directly into the combustion chamber within the cylinder has been developed in recent years, the fuel cut control is desired to be applied to such an in-cylinder injection engine.
Such an in-cylinder injection engine can inject fuel into the combustion chamber at any time regardless of the opening and closing of the intake valve and therefore can carry out various characteristic operations as follows:
For example, a fuel injection mode with a compression stroke as primary (this is referred to as a compression stroke injection mode) can be set. In this compression stroke injection mode, stable combustion can be realized in a ultra-lean air-fuel ratio state by stratified combustion making use of the stratified intake-air flow formed within the cylinder. In other words, since injected fuel can be collected near the spark plug by making use of stratified intake-air flow, operation can be performed saving fuel consumption considerably with the entire fuel-air ratio caused to be in an extremely lean air-fuel state, obtaining stable ignitability with only the vicinity of the spark plug caused to be in a good ignitable air-fuel state (i.e., stoichiometric air-fuel ratio state or air-fuel ratio state on a side slightly richer than the stoichiometric air-fuel ratio state).
Of course, a fuel injection mode with an intake stroke as primary (this is referred to as an intake stroke injection mode) can be set. In this intake stroke injection mode, stable ignition and reliable flame propagation are realized evening the entire air-fuel ratio state of the combustion chamber by premixing fuel, whereby operation can be performed so that sufficiently high output is obtainable. In this intake stroke injection mode, a stoichiometric mode in which great output is obtained by adjusting an air-fuel ratio to near a stoichiometric air-fuel ratio and a lean mode in which fuel consumption can be saved by causing an air-fuel ratio to be leaner than a stoichiometric air-fuel ratio are considered. In addition, in view of the case where great output is desired to be obtained temporarily during rapid acceleration, etc., an enrich mode in which an air-fuel ratio is richer than a stoichiometric air-fuel ratio is considered.
In such an in-cylinder injection engine, incidentally, the engine is operated by properly selecting various operating modes such as the aforementioned compression stroke injection mode (lean compression mode or late lean mode), a stoichiometric intake stroke injection mode (stoichiometric mode), a lean intake stroke injection mode (lean intake mode or early lean mode), and an enrich intake stroke injection mode (enrich mode). The selection of these operating modes is considered to be performed based on engine speed and engine load.
In other words, the lean compression mode is selected in an area where engine speed is low and engine load is small, and if the engine speed becomes higher than the low engine speed and the engine load becomes great, the lean intake mode, the stoichiometric mode, and the enrich mode will be selected in the recited order, as the degree becomes greater.
Engine load nearly corresponds to the degree of depression of the accelerator pedal, so when the lean compression mode is selected, the depression degree of the accelerator pedal is slight and therefore the throttle opening degree is slight. On the other hand, in the lean compression mode in which operation is performed in a super-lean state in which an air-fuel ratio is extremely great, if there is no sufficient intake-air quantity, the engine output at the same throttle opening degree will become small. Also, stratified flow will become weak and stable combustion will be difficult to be carried out. Therefore, if a throttle opening degree is slight and an intake-air quantity is regulated with the throttle valve, the operation in the lean compression mode will be difficult. For this reason, in the in-cylinder injection engine, the air quantity regulated with the throttle valve is supplemented, by providing a bypass passage (air bypass passage) which bypasses the throttle valve and controlling the opening degree of a valve (air bypass valve) provided in the air bypass passage.
However, if such fuel cut control employed in the conventional MPI engine, as it is, is applied to such an in-cylinder injection engine, a sudden reduction in the output torque of the engine will be caused when fuel cut is started, and in the case of a vehicle-mounted engine, a vehicle will be considerably shocked.
FIG. 9 is a diagram schematically showing the characteristics of the output torque of an engine in the case where the engine makes a transition from a non-idle state to an idle state when the engine speed is greater than a predetermined speed (fuel cut condition speed) Ne1 and then makes a transition to a fuel cut mode. In the figure, "A" represents a torque change characteristic in the case of the conventional MPI engine and "B" represents a torque change characteristic in the case of the in-cylinder injection engine. In this case, after establishment of fuel cut conditions (i.e., after the engine has gone to an idle state), the throttle valve is not fully closed suddenly but is gently closed through operation of a so-called dashpot.
As shown in FIG. 9, in the conventional MPI engine, in the circumstances in which fuel cut control is started, i.e., in the case where the throttle valve is fully closed or the engine is in an idle state (see FIG. 9(A)), irregular combustion takes place in the engine and therefore the engine output torque is already reduced easily. With the operation of the dashpot, the intake-air quantity is reduced, accordingly the fuel injection quantity is also reduced, and the engine output torque is considerably reduced. For this reason, when a transition to a fuel cut mode is made, the torque difference di becomes small as shown in FIG. 9(B) and a vehicle will not be easily shocked.
On the other hand, in the in-cylinder injection engine, in the circumstances in which fuel cut control is started, i.e., in the case where the throttle valve is fully closed or the engine is in an idle state, operation is generally performed in the lean compression mode. However, in this lean compression mode, combustion is favorably performed even with a small quantity of fuel injection. Therefore, even if the engine goes to an idle state and the dashpot is operated, the engine output torque will be difficult to be reduced. For this reason, when a transition to a fuel cut mode is made, the torque difference d2 becomes great as shown in FIG. 9(B), gives a great shock to a vehicle, and easily gives a feeling of physical disorder to the driver, etc.
Such a disadvantage of a vehicle being considerably shocked does not arise only when a transition to a fuel cut mode is made, but it also arises when a return from a fuel cut mode to a fuel injection mode is made.
In other words, even if the engine remains in an idle state, if the engine speed is excessively reduced, in the case where there is a request to increase engine torque thereafter, the engine cannot quickly correspond to the request. Therefore, even in an idle state, if engine speed is reduced to less than a predetermined speed (fuel injection return condition speed) Ne2 (Ne2&lt;Ne1), a return to a fuel injection mode will be performed.
At this time, torque fluctuation will occur by the amount of the torque difference d2 such as that shown in FIG. 9. After all, a vehicle will have a great shock (acceleration shock).
Note that if the engine goes from an idle state to a non-idle state during a fuel cut mode, of course the fuel cut mode will return to the fuel injection mode. At this time, a driver depresses the accelerator pedal to request acceleration, so even if an acceleration shock takes place, the driver will not easily have a feeling of physical disorder and this case will not be a great problem.
As previously described, such problems are conspicuous in in-cylinder injection internal engines, but, in a conventional engine (port injection engine), in order to reduce a torque shock which occurs at the time of a transition to a fuel cut mode and at the time of a return to a fuel injection mode, the techniques that cause a fuel supply quantity to be reduced gradually at the time of a transition to a fuel cut mode and cause a fuel supply quantity to be increased gradually at the time of a return Lo a fuel injection mode are disclosed, for example, in Japanese Patent Publication Nos. SHO 58-20374 and SHO 63-42098.
These techniques, however, are for port injection engines, so if the fuel injection quantity at the time of a start of fuel cut and the fuel injection quantity at the time of a return to fuel cut, disclosed in these prior techniques, are employed to control fuel cut in the compression stroke injection mode of an in-cylinder injection engine, there will be the problem that output will be excessive and therefore a torque shock will occur, as previously described. Therefore, this technical idea, as it is, cannot be applied to the in-cylinder injection engine to which the present invention is applied. In other words, the in-cylinder injection engine has a particular fuel injection mode such as the compression stroke injection mode, as previously described, and for the torque shock reduction at the time of the switching between the fuel injection mode and the fuel cut mode, a particular efficient technique is desired to be developed.
The present invention has been made in view of the aforementioned problems, and accordingly, it is an object of the invention to provide a fuel control apparatus for an in-cylinder injection internal engine which can start fuel cut control without causing a switching shock even in an in-cylinder injection internal engine. Another object of the invention is to provide a fuel control apparatus for an in-cylinder injection internal engine which can make a transition to a fuel cut mode and a transition from a fuel cut mode to a fuel injection mode without causing a switching shock, contriving air-fuel ratio control in a compression stroke injection mode in an in-cylinder injection internal engine.